2025 H2 Chem Prelim P3 Worked Solutions_CJC

9729/03/CJC JC2 Preliminary Examination 2025 [Turn over CANDIDATE NAME CLASS 2T INDEX NUMBER CHEMISTRY 9729/03 Paper 3 Free Response 15 September 2025 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 32 printed pages. For Examiner’s Use Section A Q1 /21 Q2 /20 Q3 /19 Section B Q4 /20 OR Q5 /20 TOTAL 80 Catholic Junior College JC2 Preliminary Examination Higher 2

2 9729/03/CJC JC2 Preliminary Examination 2025 Section A Answer all the questions in this section. 1 (a) Phenylalanine is an essential -amino acid with the formula C 9H11NO2. It cannot be synthesised by the human body and need to be obtained through diet. Phenylalanine can react with alanine, CH3CH(NH2)CO2H, to form a mixture of dipeptides. (i) Name the type of reaction when phenylalanine reacts with alanine. [1] (ii) State the number of possible different constitutional isomers that can be formed in this reaction. [1] (iii) Draw the structures of the constitutional isomers with the molecular formula C12H16N2O3 that are formed in this reaction. The peptide bond formed in each constitutional isomer should be shown displayed. [2] …………………………………………………….…………………………………………… … …………………………………………………….…………………………………………… … (a) (iii) Phe-Ala: (a)(i) condensation (a)(ii) 2 constitutional isomers peptide bond

3 9729/03/CJC JC2 Preliminary Examination 2025 [Turn over Ala-Phe: …………………………………………………….……………………………………………. .. (b) Table 1.1 shows the pKa values of the different functional groups present in phenylalanine and alanine. Both amino acids exist mainly as zwitterions at pH 7.0. (i) State what is meant by the term zwitterion. [1] Table 1.1 (ii) Draw the predominant species of phenylalanine at pH 1.0. [1] (iii) Suggest a pH at which the predominant speci